Laminated Veneer Products : Shape Stability and Effect of Enhanced Formability on Bond-Line Strength
Sammanfattning: This thesis concerns two aspects of the manufacture of laminated veneer products (LVPs).The first aspect is related to the possible improvement of the shape stability of LVPs, andthe second has its starting point in the modification of the veneer for enhanced formabilityas well as the question of whether and how these modifications affect the bond-linestrength.LVPs are veneers bonded with adhesive into predetermined shapes, mostly for theproduction of furniture and interior fittings. Since any deviation from the intended shape isa problem for both manufacturer and customer, various studies have sought to evaluate theinfluence of different materials and process parameters on shape stability. Parametersstudied have included wood species (beech and birch), an adhesive system based on ureaformaldehyde, the adhesive distribution on the veneer, climate, moisture content and fibreorientations of the veneers, as well as the orientation of the individual veneers in a multiply.Manufacturers of LVPs must consider some basic facts about wood in orders adequately toprovide shape-stable LVPs to customers. Wood emits and absorbs moisture in relation tothe surrounding climate, and this can lead to shrinkage and swelling. Such moistureinduced movements differ in the wood’s different directions, and the magnitude is specificfor the species. A thorough understanding of this is the basis for achieving shape-stableLVPs.Symmetry is defined in this thesis such that the veneer properties are balanced in thelaminate. This means that opposite veneers on either side of the centre veneer have similarcharacteristic. An LVP will become distorted if the veneers are asymmetrically orientedbefore the press. Deviation from the desired shape can be small immediately after thepressing, but it may increase significantly with moisture content (MC) variations.Asymmetry may result when veneers with different fibre orientations are included in thelaminate or when the veneers are placed asymmetrically. It may also occur if veneers withdifferent MCs are bonded together asymmetrically. One aggravating factor is that the lathechecks that are introduced when the veneers are peeled or sliced from the log affect theshape stability. In 3-ply crosswise-oriented plywood, the veneer surfaces on which the lathechecks occur should be oriented in the same way for high shape stability.Based on existing knowledge, the production of shape-stable LVPs requires that theveneers are conditioned to a uniform MC and sorted with regard to fibre orientation andthe side with lathe checks before bonding. End-user climates should govern the MC of theveneers and the moisture added with the adhesive during the process. Straight-grainveneers and symmetry should always be the goal.Moulding can cause stretching, i.e. strain, of the veneers depending on the curvature of themould. To prevent the veneers from rupture, there are various ways to strengthen theveneers particularly in the transverse direction in which the veneer is weakest. However,tests have shown that these pre-treatments of veneers for enhanced formability can preventthe adhesive from penetrating the wood surface. It is therefore important to confirm thatthe pre-treatment does not affect the bond-line strength.
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